In the past, various attempts have been made to improve the cutting capabilities of conventional twist drills. While some of these improvements have been accepted and incorporated into drills for use with conventional metals, high strength thermal resistant materials such as titanium, have still proven to be extremely difficult to drill, especially if one is after the capability of drilling a large number of good quality holes between resharpening.
Such metals place conflicting demands upon the drill. First, there is a need for high-torsional strength to prevent the tendency of the drill to "unwind" which causes chatter and damage to the cutting edges. Second, there is a requirement for sufficient tool body at the cutting edge to dissipate the heat generated during drilling. Conventional general purpose drills have helix angles of approximately 30 degrees, thereby providing substantial tool body at the cutting edges; but they do not have adequate torsional strength and, therefore, are prone to chattering. Greatly increasing the helix angle, to 45 degrees, for example, reduces the chattering, but it also reduces the tool body at the cutting edge. Hence, cutting-edge strength and the ability to dissipate heat are also lost. Additionally, when helix angles are increased, chip flow is impaired, causing the flutes to clog, particularly when drilling metals such as aluminum, thereby nullifying the benefits of increased torsional strength.
National Aerospace Standard 907 (hereinafter abbreviated NAS 907) sets forth present day drill design philosophy for these difficult-to-drill, high-strength metals. But drills meeting the NAS 907 dimensional standards still do not provide sufficient drill life. It has been determined that even small dimensional changes, or just the tightening of tolerances on a given dimension, can radically change the effectiveness of a drill. For example, in U.S. Pat. No. 3,387,511, "Twist Drill" by W. K. Ackart, Sr, et al. (Applicant is coinventor), slight changes in the chisel-edge angle obtained a significant increase in drill life over then existing drills. In U.S. Pat. No. 4,556,347 "Split-Point Twist Drill" by H. B. Barish, a dramatic increase in drill life is obtained with a unique combination of such small changes.
Additional patents of interest include U.S. Pat. No. 4,116,580, "All Cutting Edge Drill" by R. F. Hall, et al.; U.S. Pat. No. 2,936,658, "Twist Drill" by O. L. Riley; U.S. Pat. No. 4,222,690, "Drill Having Cutting Edge with the Greatest Curvature at the Central Portion Thereof" by R. Hosoi, and 4,065,224, "Twist Drill" by K. Siddall. While the above list of patents are felt to be of interest, they are not considered to be relevant.
When drilling holes in composites such as filamentary reinforced thermosetting or thermoplastic resins matrixes such as the combination of graphite and polyetheretherkeytone (PEEK) thermoplastic or fiberglass/thermosetting epoxy the problems are different. The graphite and glass within these materials are very abrasive and rapidly wear the cutting edge of such previously mentioned drills. Thus, special eight facet drills are used which, while somewhat successful with composites, are useless with metals.
In a "sandwich" combination, such as titanium sheet, a composite such as a graphite filament reinforced PEEK and a 13-8 stainless steel sandwich structure, conventional drills are rapidly dulled and the titanium welds thereto. Also, the conventional drills for high strength metal leave burrs at the entrance and exit of the hole due to the heavy wear on the cutting edge corners. Futhermore, conventional high strength metal drills do not provide proper chip break up and the emerging chips can cause chippage or breakout of the hole walls. Thus, the hole must be drilled undersized and thereafter reamed to final dimensions.
Therefore, it is a primary object of the subject invention to provide a twist drill that can be used to drill a structure composed of a combination of titanium, steel and resin matrix composites.
Another object of the invention is to provide a twist drill that can drill a structure composed of a combination of titanium, steel and resin matrix composites that eliminates the need to ream (depending upon hole tolerance) the hole thereafter.
A further object of the invention is to provide a twist drill that can drill a structure composed of a combination of titanium, steel and resin matrix composites providing for a greater number of holes to be drilled between resharpenings than existing drills.